20 Linux Log Files

If you spend lot of time in Linux environment, it is essential that you know where the log files are located, and what is contained in each and every log file.

When your systems are running smoothly, take some time to learn and understand the content of various log files, which will help you when there is a crisis and you have to look though the log files to identify the issue.
/etc/rsyslog.conf controls what goes inside some of the log files. For example, following is the entry in rsyslog.conf for /var/log/messages.

mail.none,authpriv.none,cron.none indicates that those error messages should not be logged into the /var/log/messages file.

You can also specify *.none, which indicates that none of the log messages will be logged.

The following are the 20 different log files that are located under /var/log/ directory. Some of these log files are distribution specific. For example, you’ll see dpkg.log on Debian based systems (for example, on Ubuntu).

/var/log/messages – Contains global system messages, including the messages that are logged during system startup. There are several things that are logged in /var/log/messages including mail, cron, daemon, kern, auth, etc.

/var/log/dmesg – Contains kernel ring buffer information. When the system boots up, it prints number of messages on the screen that displays information about the hardware devices that the kernel detects during boot process. These messages are available in kernel ring buffer and whenever the new message comes the old message gets overwritten. You can also view the content of this file using the dmesg command.

/var/log/auth.log – Contains system authorization information, including user logins and authentication machinsm that were used.

/var/log/boot.log – Contains information that are logged when the system boots

/var/log/daemon.log – Contains information logged by the various background daemons that runs on the system

/var/log/dpkg.log – Contains information that are logged when a package is installed or removed using dpkg command

/var/log/kern.log – Contains information logged by the kernel. Helpful for you to troubleshoot a custom-built kernel.

/var/log/lastlog – Displays the recent login information for all the users. This is not an ascii file. You should use lastlog command to view the content of this file.

/var/log/maillog /var/log/mail.log – Contains the log information from the mail server that is running on the system. For example, sendmail logs information about all the sent items to this file

/var/log/prelink/ – prelink program modifies shared libraries and linked binaries to speed up the startup process. /var/log/prelink/prelink.log contains the information about the .so file that was modified by the prelink.

/var/log/setroubleshoot/ – SELinux uses setroubleshootd (SE Trouble Shoot Daemon) to notify about issues in the security context of files, and logs those information in this log file.

/var/log/samba/ – Contains log information stored by samba, which is used to connect Windows to Linux.

/var/log/sa/ – Contains the daily sar files that are collected by the sysstat package.

/var/log/sssd/ – Use by system security services daemon that manage access to remote directories and authentication mechanisms.

Instead of manually trying to archive the log files, by cleaning it up after x number of days, or by deleting the logs after it reaches certain size, you can do this automatically using logrotate as we discussed earlier.

To view the log files use any one of the following methods. But, please don’t do ‘cat | more’.

vi – If you are comfortable with the vi commands, use vi editor for quick log file browsing.

tail – If you want to view the content of the log files real time, as the application is writting to it, use “tail -f”. You can also view multiple log files at the same time (using “tail -f”).

grep – If you know exactly what you are looking for in a log file, you can quickly use grep command to grep a pattern. The 15 practical grep examples will take out all your excuses of not using grep.

less – Less command is very powerful to browse log files. Use these 10 less command tips to master the less command.